In the fields of chemical processing, food preparation, waste treatment, health services, and many others, it is common to employ electro-chemical techniques to detect the presence or to determine the concentration of particular chemicals. Electro-chemical measurement is complicated by the fact that signal levels are very low, and complicated by the further fact that available sensors exhibit high impedance. In most cases it is necessary to include a liquid path in the measuring circuit to permit ion migration. The structure of the sensor is required to have high impedance and to be inert. These factors and others, including the fact that a glass membrane is used in the most common application, the sensing of pH, and the use of universal read-out instrumentation and sensor mounting structures, have led to a high degree of standardization in the configuration of ion selective electrode sensors and reference electrodes.
Most electrodes are formed as elongated cylinders in which the ion selective element, or the salt bridge in the case of a reference electrode, are located at one end of the cylinder. The electrical leads or the electrical connectors are located at the cylinder's opposite end. The most commonly used material for forming the cylindrical body is tubular glass. Glass is inert, and it bonds readily to pH sensitive glass. Even when plastics, the other popular body material, is used, the body is usually formed as an elongated cylinder.
That portion of the electrical measurement path in which ion movement occurs must form a junction with the sensor, hence it is almost universal that the liquid be contained within the tubular electrode body. The liquid is conductive, an electrolyte, and it is made conductive by the addition of a salt. To simplify correction of the result to account for junction potentials, the salt is added to saturation and connection to the liquid is made through a chemically related material. Commonly, the electrolyte is a saturated solution of KCl, and connection to the instrumentation is made through a silver wire coated with silver and silver chloride. The electrode must include a closure because it is a liquid container, and in almost all cases the instrument wire is the center conductor of a coaxial cable that extends through the closure.
What has been set out above is a description of the structural form that is produced by almost all of the manufacturers in this field which has enjoyed the greatest commercial success for the longest period, and which has long been the subject of cost reduction and performance improvement inventive effort. Despite past efforts, though, this standard, cost reduced, high performance electrode form has a severe limitation. It cannot be used upside down (selective end up) in many installations. Air bubbles in the electrolyte interrupt, or change the area of, the ion migration portion of the measurement current flow path. The obvious solution is to completely fill in the electrolyte cavity, but that does not solve the problem. During the course of manufacture, shipment and use, the electrolyte experiences a wide range of temperatures. Those temperatures may range from below zero degrees centrigrade (freezing) to above 100 degrees centigrade (boiling). The electrolyte is incompressible and it undergoes relatively large changes in volume. It is part of the standard design to include an air bubble whereby to accommodate volumetric change.